The present invention relates to a starter motor for an internal combustion engine and, particularly, to such motor which is compact and capable of providing a high reduction ratio.
FIG. 1 shows an example of a conventional d.c. starter motor, in cross section, which comprises an armature 1 including a core 1a composed of a lamination of electromagnetic steel plates and an armature coil 1b wound thereon, a rotary shaft 2 of the armature 1, a commutator 3 mounted behind the armature, a brush and holder assembly 4 fixedly secured to a rear bracket 6 by bolts 5, a bearing 7 fixed in a recess of the rear bracket 6 for supporting a rear end portion of the rotary shaft 2, a yoke 8 of the d.c. motor whose one end surface is fitted in the rear bracket 6, a permanent magnet 8a fixed on an inner peripheral surface of the yoke 8 for producing a magnetic field, and a flat gear 9 fixedly mounted on a front portion of the rotary shaft 2 and meshed with planet gears ( not shown ) of a planet reduction gear mechanism having a low reduction ratio. An output of the planet reduction gear mechanism is transmitted through an overrunning clutch ( not shown ) to a pinion ( not shown ). The rotation of the pinion is transmitted through a ring gear formed on a periphery of a fly-wheel fixedly mounted on a crank shaft of an engine to start the latter.
In such conventional starter motor, it is possible to obtain a necessary torque directly. However, due to the use of the lamination core 1a and because the temperature of the armature 1 increases in proportion 1 ( power consumption of the armature coil 1b)/(volume of the motor ), the power consumption of the armature coil 1b becomes considerable and substantial heat is generated thereby when the number of turns of the coil 1b and hence the resistance thereof is large. In order to protect the armature coil 1b from the thermal breakdown of its insulation, it is necessary to make the motor large and complicated in structure, which makes the motor expensive and provides a larger possibility of malfunction of commutator 3.
Further, due to the use of the transmission path of rotational force which includes the pinion and the ring gear, the location of the starter motor with respect to the engine is limited. Further, in order to increase the reduction ratio, it is necessary to make the ring gear on the fly-wheel large, leading a large transmission mechanism. Alternatively, it may be possible to reduce the number of teeth of the pinion. However, since the maximum reduction ratio is 120/8=15 at present, it is necessary to provide another reduction device for a reduction ratio larger than 15.